1. Field of the Invention
The present invention relates to a circuit for detecting the failure of a step motor to respond to energization commands.
2. Description of the Prior Art
It is known that step motors are widely used in electromechanical applications where incremental displacements of movable elements are required. They are particularly used in high speed printers. It is also known that these types of motors can be controlled by open loop control systems or closed loop control systems. The closed loop control systems comprise sensing means coupled to the rotor of the motor which means supplies a feedback signal. Among the sensing means, tachometric dynamo and photodisks are typical. The photodisks supply speed and position signals and therefore detect the step motor's responses to the energization commands it receives. Also in the case of a tachometric dynamo, the signal generated by such means enables the detection of a step motor's responses to the energization commands it receives. U.S. Pat. No. 3,896,363 discloses, for example, a feedback circuit which uses a tachometric dynamo for detecting whether or not a step motor responds to the energization commands. However, because of the high costs of closed loop control systems, open loop control systems which do not require sensing means are increasingly used.
It is desirable therefore that these types of control systems are provided with means for detecting the failure of step motors to respond to energization commands. For example, in a serial printer, the step motors are used both for moving the printing carriage along the printing line and for advancing or feeding the printing support. The failure of the motors to respond to energization commands, which may be due to extraneous resisting torques, causes problems in printing and may result in overlapping characters in the same printing position or the printing of several overlapping rows along the same line of the printing support. Generally such problems are detected later by the operator or by the printer itself at preestablished intervals. For example, the serial printers are generally provided with microswitches at the end and beginning of the printing carriage stroke. By counting the energization commands sent to the printing carriage motor, it can be determined whether or not the right number of commands were sent but only at the end of the carriage stroke. Obviously the limitations of such approach are clear. What is desired is to detect any lack of response of a step motor in real time, that is, when the incorrect operation occurs. The scope of the present invention is to overcome such limitations by supplying a circuit for detecting the failure of a step motor to respond to energization commands. Such a circuit can be used in open loop control systems and supplies an error signal in real time.